1. Field of the Invention
The present invention relates to photocopying and is particularly concerned with improvements in photocopying material and the method of its manufacture.
2. Description of the Prior Art
The basic principle of the current, widespread xerographic copying method (electrophotography) lies in initially generating a latent, electrical charge image of the characters to be transferred on and intermediate carrier, in electrostatically agglomerating colorant particles to the latent image and, finally, in transferring the colorant onto the ultimate carrier (paper) and, for example, fixing it thereon by heat.
The intermediate carrier then is applied on a planar electrode (plate or drum) can be constructed of one or more layers. Given the single-layer structure, the following requirements are made of this layer;
1. An adequate photoconductivity must be present in order to be able to convert light signals into electrically-acquirable pulses. Values above 10.sup.-5 cm.sup.2 /Vs are adequate for the mobility of the charge carriers and values greater than 10.sup.-2 electrons/photons are adequate for the intrinsic quantum yield; PA0 2. The dark conductivity must be below 10.sup.-10 (ohm meter).sup.-1 and the dielectric strength must be greater than 100 V/.mu.m so that the unexposed layer can be electrostatically charged (corona discharge) and the charges are prevented from undesirably flowing off; PA0 3. No cross diffusion or only slight cross diffusion of the surface charges should be present during the copying process in order to avoid unsharpness of image or errors. This requires an adequate number of localized conditions (adhesion locations) in the surface; PA0 4. The material must have a high thermal conductivity in order to oppose excessive heating due to light absorption and also in order to suppress potential thermoelectric effects; PA0 5. A high resistance to wear vis-a-vis the paper bands running during copying should be present, at least in the surface region; PA0 6. The material should be uniform, i.e. should have amorphous structure; PA0 7. The material should be moisture-insensitive in order to avoid effects of tracking currents through the layer; PA0 8. The material should have no toxic effects; PA0 9. The manufacture should be possible in a large-area process and should be simple to execute; and PA0 10. The material should have an optical band spacing Eg below 3 eV in order to absorb the visible range. PA0 is applied in a layer format on a plate-shaped or drum-shaped substrate; PA0 comprises a photoconductive layer; and PA0 wherein at least the uppermost layer is composed of amorphous, hydrogen-containing carbon (a--C:H).
As may be derived from the book by W. Heywang, "Amorphe und polykristalline Halbleiter", Springer Verlag, Berlin, 1984, pp. 40-46, thin, polycrystalline or, respectively, amorphous, toxic selenium layers are still currently used in 90% of cases as electrically chargeable, photoconductive intermediate image carriers for electrophotographic processes, these selenium layers being distinguished by favorable transport properties for optoelectrically generated charge carriers and by low cross diffusion of the charges. What is thereby advantageous is that photoconduction can only be achieved with shortwave physiologically harmful light below 400 nm and that the layers are mechanically sensitive and can usually be photoelectrically employed only in combination with thin cadmium electrodes.
The utilization of amorphous, hydrogen-containing silicon layers (a--Si: H layers) as latent intermediate image carriers in the electrophotographic copying process is also known (Heywang suora, p. 47). The material a--Si:H has a high photoconductivity but it is usually only utilized as a charge-generating layer in combination with another layer that guarantees a better charge transport, for example, a--C:H material, as disclosed, for example, in the European application 0 250 910. Since it is moisture sensitive as a result of thin oxide layers and is also mechanically stable only to a limited degree, A--Si:H layer must usually already be covered by a thin protective layer.
The method of layer combination as disclosed, for example, in the European Patent Application 0 250 916 or in the German patent 3,201,146 C2 is very involved in technological terms.
An extensive listing of possible layer combinations and their conceivable embodiments may be derived from the German published application 36 31 350. An electrophotographic recording material disclosed therein can comprise up to five different layers lying on top of one another, these being applied over an electrically-conductive substrate. In this arrangement, a separate function is assigned to each layer, so that the requirements set forth above no longer need be met by a single layer. Therefore, for example, a barrier or adhesion layer can be initially provided directly over the substrate. A charge transporting layer is provided between the charge generating layer and the adhesion layer in order to enable the charge carriers generated in the exposure to flow off to the substrate. A further charge transport layer above the mentioned charge generating layer enables the transport of charge carriers of the other type to the uppermost layer in order to neutralize the electrostatic charging seated there at the unexposed locations.
This uppermost layer serves as a barrier and a protective layer and is the carrier of the electrostatic charge. At the same time, it must also withstand the mechanical demands in the printing process over a long term.
As likewise known from the book of W. Heywang, Suora, p. 47, organic polymers are utilized as photoconductors in electrophotography in addition to the mentioned amorphous semiconductor materials, most often charge carrier complexes of trinitrofluorene (TNF acceptor) and polyvinyl polyvinylcarbazole (PVC donor). These systems can be cost-effectively manufactured in a large-area manner; however, their charge carrier mobility is extremely low and their mechanical stability and the thermoconduction are only slight.